The present invention generally relates to ophthalmic procedures and more specifically relates to methods for vision correction.
Refractive errors occur when light rays entering the eye are not focused precisely on the retina. In nearsightedness (myopia), light is focused in front of the retina. In farsightedness (hyperopia), light is focused behind the retina. In astigmatism, light is focused in more than one focal point.
Most people with astigmatism have “regular” astigmatism. Regular astigmatism usually occurs when the cornea (the front surface of the eye) has an oval (or toric) curvature rather than round curvature. In other words, one dimension (meridian) of the cornea is steep, and one dimension is flat.
“Irregular” astigmatism and “higher order aberrations” occur when the eye's focusing elements are neither round nor uniformly oval but irregular in their curvature. For example, the eye of a person with a thickened scar on one side of the cornea would have irregular focusing properties.
Conventional vision correction for astigmatism includes toric spectacles, soft contact lenses and rigid contact lenses. Rigid contact lenses are known to work well for many types of refractive error, including irregular astigmatism. Corrective lenses, however, are cumbersome, uncomfortable, and may present certain risks to the health of the cornea.
Conventional surgical vision correction includes radial or astigmatic keratotomy which involves deep incisions into the cornea to correct vision. These techniques have limitations in accuracy and in the magnitude of correction that can be achieved. Additionally, the incisions may mechanically weaken the cornea, compromise the structural integrity thereof, and cause the cornea to be more susceptible to injury, particularly in the case of a patient with a relatively large pupil size.
Excimer laser vision correction procedures (photorefractive keratectomy and lasik) require the use of expensive equipment and involve the permanent removal of corneal tissue, which can lead to corneal weakening. In lasik procedures, surgeons create a thin (130-160 micron) hinged “flap” of corneal tissue that is moved from the center of the cornea to expose its inner layers. Laser treatment is applied to the inner corneal layers in order to reshape the curvature. The corneal flap is then replaced in its original position and allowed to heal. During the surgical procedure, the flap is designed to remain attached to the cornea by its hinge.
The prior art has also included various corneal implants and other implants for vision correction, including but not limited to those described in U.S. Pat. Nos. 6,893,461; 6,702,807; 6,623,522; 6,607,566 and 6,361,560, each of which is expressly incorporated herein by reference. Additionally, the prior art has included various corneal “onlays” which promote overgrowth of corneal epithelium, including but not limited to those described in U.S. Pat. Nos. 6,689,165; 6,454,800 and 5,713,967, each of which is expressly incorporated herein by reference.